Device for producing pressed articles

ABSTRACT

A device for producing pressed articles with a main cylindrically-shaped elements and secondary helically-shaped element from powdered material, especially powdered metal, with a female mold and with at least one top punch, which is supported so as to be rotatable about its longitudinal axis, and at least one bottom punch, which is supported so as to be rotatable about its longitudinal axis. The top and bottom punches are movable axially relative to the female mold by means of motor drives and the top punch, of which there is at least one, is driven in rotation about its longitudinal axis to achieve a helical movement in addition to its axial drive the bottom punch, of which there is at least one, is also driven in rotation. Rotary motor drives of the punches are mechanically uncoupled from the drives for the axial movement of the punches and may be regulated separately by an electronic control.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a device for producing compacted or pressedarticles with a main cylindrically-shaped element and a secondaryhelically-shaped element from powdered material such as powdered metal,and in particular for producing helical gearwheels in which the helicalgearing or toothing is the secondary element.

2. Description of the Related Art

A conventional device as described in European Patent Publication 0 528761 A1 produces pressed articles, e.g., helical-toothed gearwheels, frommetal powder. This known metal powder press has a linearly movable upperram in which is supported a top punch which is rotatable about thelongitudinal axis in the pressing direction and a bottom ram which isalso moveable linearly against a bottom stop and in which a bottom punchis rotatably supported. A die plate forms a mold cavity and is movablelinearly in the pressing cycle. The rotatable bottom punch and therotatable top punch each have a toothing profile corresponding to thetoothing profile or helical toothing of the mold shell or casing, i.e.,the mold cavity. The bottom punch which is supported so as to be freelyrotatable is constantly engaged with the profile of the mold cavity andtherefore rotates compulsorily in a corresponding manner when linearrelative movements occur between the bottom punch and die plate duringthe press cycle. In contrast, a rotational movement corresponding to thehelical toothing is externally forced upon the top punch during thepress cycle corresponding to its penetration depth in order to reducethe friction between the outer surfaces of the top punch and mold cavityof the female mold. The toothed-wheel mechanism provided for thispurpose is driven via a mechanical linkage control corresponding to thedesired helical toothing of the pressed article. The linkage controlcontains linkage cores, rigidly connected to a guide plate and guided ina positive engagement and in a sliding manner in the coaxially arrangeddriving wheels of the toothed-wheel mechanism. During the press cycle,the guide plate is temporarily rigidly coupled with the die plate andmoves jointly therewith. A withdrawal process is used to remove theproduced pressed articles from the mold.

This known metal powder press gives rise to considerable costs withrespect to mechanical construction and also retooling since for everydifferent pressed article a set of linkage cores corresponding to thispressed article must be prepared and exchanged, aside from the specialtool set including the female mold, top punch and bottom punch. Added tothis is the cost of the guide plate and the mechanically operatedlocking device for rigidly coupling the guide plate to the die plate.There also remains the problem of friction between the bottom punch withrespect to its rotational movement and the female mold, the bottom punchnot being positively driven externally. This not only results inincreased tool wear in this region, but also leads to an uneven densitydistribution in the pressed article.

A press with electronically controlled movements which is used for therotary press process is described in the publication entitled "Qualitycontrol through process monitoring of rotary forming press", MetalPowder industries Federation, Volume 6, May 6-11, 1994, 125-137. A pressof this type is used for subsequent treatment of already sintered moldedarticles produced by powder metallurgy in order to give them a densityin the range of 95% to 98% of the theoretically possible density of thematerial in question. The special construction of these presses makes itpossible to generate extremely high local pressing pressures in thepressing tool with a comparatively low overall pressing force of thepress. The special construction for this purpose includes an upper punchdie that moves in a gyrating and rotating manner and applies locallydefined extremely high pressing forces on the workpiece in order tocompact the latter in directed manner. This reference contains nosuggestion that the top punch and bottom punch, which participatedirectly in the shaping of the helically shaped secondary element of thearticle to be pressed, may be controlled with respect to their movementin the mold cavity of the female mold by electronic means for thepurpose of producing compact with main cylindrically shaped elements andsecondary helically shaped elements from powdered material.

SUMMARY OF THE INVENTION

The object of the invention is to improve a generic device in such a waythat the friction problems mentioned above with regard to the bottompunch are solved satisfactorily, while the tool cost and expense ofretooling for production of different pressed articles remains as low aspossible.

A substantial feature of the invention consists in that the top punchand bottom punch which directly participate in the shaping of thesecondary helically shaped element of the pressed article to beproduced, for example, in gearwheels with a plurality of toothingsarranged axially in succession, a corresponding plurality of bottom ortop punches may be required are guided by an electronic control withrespect to their movement in the mold cavity of the female mold. Therotational movement of the bottom punch and top punch, depending uponthe depth to which the latter penetrate into the mold cavity, is ensuredby an electronically regulated, separate i.e., mechanically uncoupledrotary drive and is thus no longer effected by the mechanical couplingof a linear and rotary drive, i.e., no longer exclusively by thefriction between the punch contour and the outer surface of the femalemold as is the case with the bottom punch shaping one broad side in thepress as described in European Patent Publication EP 0 528 761 A1, or bymechanical sensing of a linkage core as is the case with the top punchin this known press which shapes the other broad side. This means thatthe simultaneous rotational movement corresponding to the lead or pitchof the secondary helically shaped element and the linear movement mustbe effected with an accuracy lying within the tool play of the toothingbetween the punches and the female mold. Accordingly, the axial androtational movement of the punches is effected in a position closed loopcontrolled regulated manner. Suitable sensors are provided fordetermining the respective axial and rotational position e.g., linearpotentiometer or incremental measurement means for linear movement androtational angle transmitters for rotation. In this regard, it will benoted that the female mold is advisably held in a stationary manner inthe utilized powder press while the top punch and bottom punch are movedlinearly and rotationally. Of course, modifications are also possible inthe sense of a kinematic reversal, e.g., in that the female mold may beheld so as to be rigid with respect to rotation but accompanies thelinear movement, as in the press discribed in European PatentPublication EP 0 528 761 A1, wherein the bottom punch is rotated inplace, while the top punch is moved linearly and rotationally. Inprinciple, the female mold could also be moved in rotation. This may bea meaningful addition to the rotational movability of the bottom punchand top punch or bottom punches and top punches when producing, e.g.,multiple gearwheels, that is, pressed articles with a plurality oftoothings of various width or with different helix angles disposed oneafter the other axially. In a known manner, core punches may also beprovided in addition to the bottom punches and top punches. These corepunches may moved by auxiliary drives and form a hub at a toothed wheelso that the toothed wheel may be placed on a shaft.

The electronic control for the movement sequence of the top punch or toppunches may be so arranged that only a purely linear movement takesplace outside of the mold cavity of the female mold and the requiredrotational movement in the press cycle is initiated immediately with thepenetration into the mold cavity, and not until then. In this case, itis advisable to provide the top punch or top punches with acomparatively soft, resilient bearing which is defined by a stop inorder to allow sufficient time for the acceleration process whenstarting the rotating movement. Position and torque are not regulateduntil the top punch has moved to the stop. This prevents damage to thetool during penetration.

Position regulation of the movement sequences is not always necessaryfor carrying out the present invention. As an alternative or in additionto the position regulation, the rotational movement sequences of the toppunch and bottom punch relative to the female mold may be set in apredetermined manner, e.g., torque values are constant with respect totime. The rotary drives of the top punch and bottom punch are adjustedin such a way in the press phase that the tooth faces of the punches andmold shell contact one another as far as possible only on the side whichwould not otherwise be exposed to a direct mutual friction in theabsence of a rotary drive when the punches move into the mold cavity ofthe female mold. Thus, the externally applied torque acts in thedirection of the rotating movement compelled by the mold. In a preferredembodiment, the torques of the rotary drives may be regulated, i.e.,increased as the pressing force increases, as a function of the pressingforce actually achieved or as a function of the achieved penetrationdepth, i.e. axial position of the punches. It is particularlyadvantageous to measure the torsional moment at the punches and to setthe driving torque precisely to a value at which the friction losses inthe bearing and drive system are compensated, i.e., the torsional momentat the punches approaches zero. The rotary drives of the punches arepreferably completely shut off in the very last segment x of the pressphase, i.e., shortly before reaching the end position of the press, inorder to prevent cracks due to torsional stresses. At a helix angle β ofthe toothing, this segment x must meet the following condition:

    x≦s·cot β

The value s equals the transverse tool play. Accordingly, a 30-degreetoothing and a tool play of 0.03 mm, for example, produces a segmentx≦0.05 mm.

In principle, the pressed article which is produced may be ejected fromthe mold by the ejection method without switching on rotary drives.However, the rotary drives are preferably used in a correspondingpartially reverse manner compared with the compacting process. Thisensures the gentlest possible treatment of the tool and pressed article.

Hydraulic drives should be used for the linear movement of the top punchand bottom punch or the moving female mold, as the case may be. Therotary drives may likewise be actuated hydraulically and in many cases,may also be operated pneumatically. Electric-motor rotary drives arepreferred, especially electric stepping motors or servomotors. Theinvention is preferably used in connection with a CNC powder press. Inanother embodiment or modification the substantially mechanical parts ofthe device according to the invention such as the punches, female moldand rotary drives are designed as an exchangeable unit in the form of atool adapter so as to enable especially short retooling times.

The present invention is advantageous in that it enables themanufacture, for example, of helical gearwheels with extremely low toolwear, since the friction in the region of the tooth flanks may belimited to a minimum during the pressing phase as well as the whenejecting from the mold. The retooling cost may also be considerablyreduced compared with known pressing tools, since only programming stepsneed be carried out rather than the manufacture of linkage cores toeffect the rotary driving. Further, when the respective powder press isprovided for producing a plurality of different pressed articles, as istypical this leads to substantial savings on investment costs in spiteof the extra expenditure required for the rotary drives and themeasuring and regulating means. Finally, it should be emphasized thatthe device according to the present invention produces pressed articleshaving an appreciably more uniform density distribution than washeretofore possible.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of the disclosure. For a better understanding of the invention, itsoperating advantages, and specific objects attained by its use,reference should be had to the drawing and descriptive matter in whichthere are illustrated and described preferred embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

The FIGURE is a schematic top view of a metal powder press in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The hydraulic press shown in FIGURE includes a press frame 16 which isoutfitted with an upper ram 14 and a bottom ram 15. A female mold 1 issupported approximately in the center of the press frame 16 so as to bestationary and rigid with respect to rotation. A top punch 2 isrotatably supported in the upper ram 14 and a bottom punch 3 isrotatably supported in the bottom ram 15. The top punch 2 is supportedso as to be flexible in the pressing direction by means of apretensioned spring 9 which presses the receiving device for the toppunch 2 in the pressing direction against a fixed stop. The force of thespring, whose travel is defined by a stop, is always appreciably lessthan the maximum pressing force for the tool, so that the top punch isat the stop before the start of the pressing process. The casing orshell of the female mold 1 and the two punches 2, 3 have a helicaltoothing corresponding to one another. Two path measurement systems 10,11 are provided for a highly precise determination of the respectivelinear position of the two rams 14, 15 and, accordingly, also of thepunches 2 and 3 connected therewith. These path measurement systems 10,11 can be constructed, for example, as incremental measuring means orlinear potentiometer. A rotary drive 5 and 7 e.g., an electricservomotor is attached to each of the two rams 14, 15, its actualrotational angle position being determined continuously by a combinedangle and torque measurement system 12 and 13, respectively. The punches2, 3 have measurement devices 12a, 13a for detecting the actualtorsional moment. The two rotary drives 5, 7 are connected, via a spurgear unit, with one of the two punches 2 or 3 in a manner consistentwith drive engineering. Two servo-proportional valves 4, 6 are providedfor positioning the two rams 14, 15 of the press. The press shown in theFIGURE includes an electronic control 8 enclosed in dashed lines whichis designed hierarchically in itself and is formed of a CNC mainprocessor 21, a regulating unit 19 and 20 for the linear movement of theupper ram 14 and bottom ram 15 and a regulating unit 17 and 18 for therotational movement of the two punches 2 and 3. An input/output unit ofthe CNC main processor for controlling and setting up the press via theoperator is designated by 22. The data processing link between theelectronic component units and sensors is shown by corresponding arrows.The two regulating units 19, 20 are subordinate to the CNC mainprocessor 21 and the regulating units 17, 18 for the rotational movementare subordinate as slaves to one of the two regulating units 19 or 20for the linear ram movement. In the present example, the regulating unit19 is subordinate as master to the regulating unit 20 submaster, i.e.,the movement of the upper ram 14 and the movement of the top punch 2, isused as a guidance variable for the movement sequence. The combinedangle and torque measurement system 12 together with the rotary drive 5and the regulating unit 17 forms a closed loop, whereby the regulatingunit 17 receives its reference value from the regulating unit 19 of thelinear drive of the ram 14 corresponding to its current axial positionas determined by the path measurement system 10. This holds true in acorresponding sense for the structural component units for the drive ofthe bottom punch 3 which operate in the same way, its linear positionbeing regulated as a function of the linear position of the top punch 2.The CNC main processor 21 takes over higher-ranked regulating andcontrolling functions and the processing of the preset data for therespective part to be produced. A dashed box inside the press frame 16indicates the main functional parts of the device according to theinvention in the form of a tool adapter which may be combined in aneasily exchangeable structural component unit that is connectable withthe top and bottom rams 14, 15.

The press of the present invention operates in the following manner:After a pressed article is removed from the mold, the bottom punch 3 ismoved down into the filling position in a position closed loopcontrolled manner based on the actual values determined by the pathmeasurement system 11 and the angle measurement system 13 correspondingto the helix angle of the helical gearwheel to be produced. Inparticular, adjustment of position of the punch is achieved by detectingthe actual position using the path measuring system 10 and torsionalmeasuring system 12, comparing the detected position with the intendedposition, and adjusting the position accordingly based on thedifference. The bottom punch 2 remains in the mold cavity of the femalemold 1. The top punch 2 is located above the mold cavity. After the moldcavity is filled with powdered steel, the top punch 2 is moved down in aposition closed loop controlled manner by means of the regulating unit19 based on the data from the path measurement system 10. At the sametime, a coordinated rotating movement of the top punch 2 is initiatedvia the regulating unit 17, the angle measurement system 12, and therotary drive 5, so that the relative rotational position of the toppunch 2 with respect to the toothing contour of the mold cavity of thefemale mold 1 allows the top punch 2 to penetrate into the mold cavitywithout making contact. This is the start of the actual pressing phasein which the introduced powdered steel is compacted. For this purpose,the bottom punch 3 and the top punch 2 are moved into the mold cavity inopposite directions simultaneously while the female mold 1 remainsstationary. The rotary drives 7 and 5 ensure a minimum of frictionbetween the punches 2, 3 and the female mold 1. Since the two anglemeasurement systems 12, 13, as combined instruments, are also set up todetect the driving torque, a regulation of torque can also be effecteddepending on the axial position of the punches 2, 3 in addition to, orin lieu of, the position regulation of the rotary drive 5, 7. Afterreaching the end position of the press, the drive system of the toppunch 2 is reversed to remove the produced pressed article from themold, i.e., the top punch 2 is moved out of the mold cavity linearly androtationally in a position-regulated manner corresponding to the contourof the pressed article. At the same time, the bottom punch 3 is likewisemoved upwards correspondingly in a position closed loop controlledmanner until its upper end face is flush with the top of the female mold1 and the pressed article is accordingly released ejection process. Thepressed article can be purposely held under a desired pressing loadduring the ejection process.

The invention is not limited by the embodiments described above whichare presented as examples only but can be modified in various wayswithin the scope of protection defined by the appended patent claims.

We claim:
 1. A system for producing a pressed article with a maincylindrically-shaped element and a secondary helically-shaped elementfrom powdered material, comprising:a press frame; a female mold disposedwithin said press frame and defining a mold cavity; at least one toppunch disposed within said press frame for operative helical rotationalmovement about its longitudinal axis and axial movement relative to saidfemale mold; at least one bottom punch disposed within said press framefor operative rotational movement about its longitudinal axis and axialmovement relative to said female mold; electronic rotary motorsoperatively connected for rotatingly driving respective said top andbottom punches; electronic axial motors operatively connected foraxially driving respective said top and bottom punches, said axialmotors being uncoupled from said rotary motors; and an electroniccontrol device operatively connected to drive and independently regulatesaid rotary and axial motors, said electronic control device controllingthe axial and rotational movements of said top and bottom punches in themold cavity of the female mold so as to directly shape the helicallyshaped secondary element of the article to be pressed.
 2. The system ofclaim 1, wherein said electronic control device is operatively adaptedto control said top punch so that it is axially displaced to penetrateinto the mold cavity of the female mold without said top punch beingrotated.
 3. The system of claim 1, further comprising a springoperatively displaced in an axial direction by said top punch.
 4. Thesystem of claim 1, wherein said electronic control device is operativelyadapted to control the rotational movement of said top and bottompunches during compacting of the article to be pressed in a positionregulated manner.
 5. The system of claim 1, wherein said electroniccontrol device is operatively adapted to control the rotational movementof said top and bottom punches during compacting of the article to bepressed based on predetermined torque values.
 6. The system of claim 5,wherein said electronic control device controls the rotational movementof said punches at a constant rate over time.
 7. The system of claim 4,wherein said electronic device controls the rotational movement of saidpunches during compacting of the article in a first direction and duringejection, after the article has been pressed, in a corresponding reversedirection.
 8. The system of claim 1, wherein said axial motors arehydraulic motors.
 9. The system of claim 1, wherein said rotationalmotors are hydraulic motors.
 10. The system of claim 1, wherein saidrotational motors are pneumatic motors.
 11. The system of claim 1,wherein said rotational motors are electrical stepping motors.
 12. Thesystem of claim 1, wherein said rotational motors are servomotors. 13.The system of claim 1, wherein said electronic control device includes aCNC main processor.
 14. The system of claim 1, wherein said female mold,said top punch and said bottom punch comprise a tool adapter.
 15. Thesystem of claim 1, wherein said female mold is rigidly supported in saidpress frame.